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Perovskite solar cells (PSCs) have witnessed a meteoric rise in device performance. However, maintaining photostability, particularly under thermal stress, remains a challenge due to defect formation in the perovskite layer. This study introduces europium (III) trifluoromethanesulfonate [Eu(OTF)3] as a multifunctional additive in two‐step processed perovskite films, significantly improving the performance and high‐temperature photostability of n‐i‐p PSCs. Density function theory (DFT) calculations reveal that the OTF− anion strongly coordinates with Pb2+, effectively inhibiting iodine vacancy defect formation. The addition of Eu(OTF)3 promotes perovskite grain growth to ≈2 µm and enhances film crystallinity. PSCs with a structure of ITO/SnO2/perovskite/PDCBT/NiOx/Au achieve a champion power conversion efficiency (PCE) of 23.8%. Under 85 ± 5 °C and 1 sun illumination at an open circuit in ambient air, the PSCs with Eu(OTF)3 retain 82% of their initial PCE after aging for 1500 h. [ABSTRACT FROM AUTHOR]

In recent years, self‐assembled monolayers (SAMs) have been proven to work efficiently as hole‐selective materials in both organic solar cells (OSCs) and perovskite solar cells. Although competitive performances are reported with these materials, a mechanistic understanding on device stability remains elusive. This study reveals that while various SAM molecules can increase the indium tin oxide (ITO) work function versus vacuum, they may not consistently result in monolayers that ensure simultaneous improvement in performance and operational stability of devices. Energetically, achieving alignment between the work function of the SAM‐modified electrode and the ionization energy (IE) of the donor is shown to be crucial for a low hole injection barrier, irrespective of the SAM's IE. Light‐induced degradation in the widely used SAM, (2‐(9H‐carbazol‐9‐yl)ethyl) phosphonic acid (2PACz), is identified through diverse aging tests and comprehensive chemical and electronic characterizations. This degradation involves SAM molecule decomposition and chemical reactions with the photoactive layer, contributing further to device degradation. Addressing these challenges, sputtered nickel oxide/SAM bilayers are proposed as hole‐selective contact with tailored interface energetics for both efficient and photostable OSCs, offering a promising alternative to commonly used hygroscopic PEDOT:PSS in OSCs. [ABSTRACT FROM AUTHOR]